1. Field of the Invention
The present invention relates generally to a method and apparatus for generating training sequence codes in a communication system, and more particularly to a method and apparatus for generating training sequence codes in a GSM (Global System for Mobile Communication)/EDGE (Enhanced Data Rates for Global Evolution) RAN (Radio Access Network) (“GERAN”) system.
2. Description of the Related Art
The 3GPP (3rd Generation Partnership Project) TSG (Technical Specification Group)—GERAN Standards Conference is working on GERAN evolution for performance improvements, such as an increased data transmission rate, high spectral efficiency, etc. As such, high-order QAM (Quadrature Amplitude Modulation) schemes including 16-QAM and 32-QAM have been added to the conventional modulation schemes including GMSK (Gaussian Minimum Shift Keying) and 8-PSK (Phase Shift Keying) in order to improve downlink performance and uplink performance.
Also, in order to increase a data transmission rate and spectral efficiency, a new symbol transmission rate of 325 ksymbols/s has been added to the existing symbol transmission rate of 270.833 ksymbols/s. The new symbol transmission rate (1.2 times as high as the existing symbol transmission rate) is applied to both downlink and uplink, and will likely be reflected in the GERAN standards in the latter half of 2007.
As mentioned above, GMSK and 8-PSK are used as modulation schemes in the conventional GERAN system. The GMSK scheme, which is a scheme for limiting the bandwidth of binary data by passing the binary data through a Gaussian low-pass filter and then performing frequency modulation with a certain deviation ratio, allows an interval between two frequencies to continuously vary, thereby achieving superior spectral convergence and high out-of-band spectrum suppression. The 8-PSK scheme, which is a scheme for modulating data in such a manner as to correspond to a phase-shifted code of a carrier, can increase frequency efficiency. There are nine techniques for packet data traffic channels (PDTCH) that are defined as coding schemes used in the EDGE/EGPRS system. The nine techniques include nine modulation and coding schemes (MCSs) for EDGE/EGPRS, MCS-1 to MCS-9. MCS-1 to MCS-4 each use the GMSK modulation scheme, and MCS-5 to MCS-9 each use the 8-PSK modulation scheme. In actual communication, one of various combinations of the modulation schemes and the coding techniques is selected and used. The MCS scheme used in transmission is determined by estimated channel quality.
FIG. 1 illustrates a structure of a downlink transmitter in a conventional GERAN system.
Referring to FIG. 1, a Radio Link Control (RLC) packet data block (RLC block) is forwarded to a channel encoder 110. In the channel encoder 110, the RLC block is encoded with a convolutional code, is punctured according to a defined puncturing pattern, and then is forwarded to an interleaver 120. The interleaver 120 interleaves the data, and forwards the interleaved data to a multiplexer 140 for data allocation to physical channels. In addition, RLC/MAC header information, an uplink state flag (USF), and a code identifier bit 130 are also forwarded to the multiplexer 140. The multiplexer (burst mapper) 140 distributes the collected data to four normal bursts, and allocates the respective bursts to timeslots of a TDMA (Time Division Multiple Access) frame. The data in each burst is modulated through a modulator 150, and then is forwarded to a training sequence rotator 160. The training sequence rotator 160 adds a training sequence code (TSC) to the modulated data, performs phase rotation for the data with the TSC added thereto, and then forwards the phase-rotated data to a transmitter 170. Units additionally needed for transmitting the modulated signal, for example, a digital-to-analog (D/A) converter, are well known to those skilled in the art, so a detailed description thereof will be omitted herein.
FIG. 2 illustrates a structure of a downlink receiver in a conventional GERAN system.
Referring to FIG. 2, transmitted bursts are received at a radio front-end stage 210 through a receive antenna. The received data is forwarded to a training sequence derotator 220 and a buffering and derotation unit 260, and the buffering and derotation unit 260 performs buffering and derotation. A modulation scheme detection and channel estimation unit 270 detects a modulation scheme and estimates channel information by using data output from the buffering and derotation unit 260. The training sequence derotator 220 performs phase derotation for the received data in a manner corresponding to the operation in the training sequence rotator 160 of the transmitter of FIG. 1.
An equalizer block 230 equalizes and demodulates the phase-derotated data, based on the detected modulation scheme and the estimated channel information, and then forwards the equalized and demodulated data to a deinterleaver 240. The deinterleaver 240 deinterleaves the data output from the equalizer 230, and then forwards the deinterleaved data to a channel decoder 250. The channel decoder restores the data forwarded thereto.
FIG. 3 illustrates a structure of a normal burst used in a conventional GERAN system.
As illustrated in FIG. 3, in transmitting data in the conventional GERAN system, a TSC consisting of 26 or 31 symbols is located in the middle of the normal burst. The TSC standards define eight types of TSCs, which are actually used in networks and terminals, and one and the same TSC is allocated within one cell. A TSC is used in an equalizer that estimates radio channel state information to remove noise and interference included in a received signal in a receiver. Also, the receiver measures channel quality or link quality from the TSC, and reports it to a transmitter, thereby enabling the transmitter to perform link quality control (LQC).
A conventional TSC consists of codes that are excellent in autocorrelation properties. Accordingly, the conventional TSC shows good characteristics when channel estimation is performed for one channel without considering inter-channel inference (ICI). In general, a cell structure in a cellular system is designed such that carrier frequencies are reused at a sufficient distance by considering co-channel interference (CCI). However, as the frequency of reuse of carrier frequencies increases, CCI also increases, which results in a significant influence on the performance of channel estimation and signal detection. Therefore, in a cellular system, such as GSM, it is preferred that channel estimation is accurately performed using a joint channel estimation method when there is heavy CCI. The performance of the joint channel estimation method is greatly affected by cross-correlation properties between TSCs. However, since TSCs in use in GERAN have been employed using a design scheme in which cross-correlation properties are wholly disregarded, system performance deteriorates in a CCI environment, and additionally lowering of system performance may be caused when the conventional TSCs is extended and applied to high-order QAM schemes including 16-QAM and 32-QAM, which are employed in a GERAN evolution system.